1. Field of the Invention
This invention relates to iron chelate compositions.
Iron chelate compounds include the compounds which contain the anions [Fe (III) EDDHA].sup.- and [Fe (III) EDDHMA].sup.-, together with a cation, for example Na.sup.+ or K.sup.+. [Fe (III) EDDHA].sup.- is the chelate anion formed between ferric iron and the quadrivalent anion of ethylenediamine-N,N'-di-(o-hydroxyphenyl acetic acid), ethylenediamine-N,N'-di-(p-hydroxyphenyl acetic acid) or ethylenediamine-N-o-hydroxyphenyl acetic acid-N'-p-hydroxyphenyl acetic acid. Commercially-available materials often contain mixtures of the ortho, para and ortho-para isomers mentioned. Although the chelating powers of the three isomers may vary considerably, the expression "iron EDDHA" as used herein covers any compound which contains, in addition to a cation or two or more different cations, a monovalent anion in which ferric iron is chelated by the EDDHA anion in any one or more of its isomeric forms, and mixtures of such compounds. [Fe (III) EDDHMA].sup.- is the chelate anion formed between ferric iron and the quadrivalent anion of either ethylenediamine-N,N'-di-(2-hydroxy-4-methylphenyl acetic acid) or ethylenediamine-N,N'-di-(2-hydroxy-3-methylphenyl acetic acid). The chelating powers of the isomers are again not necessarily equal but the expression "iron EDDHMA" as used herein covers any compound which contains, in addition to a cation or two or more different cations, a monovalent anion in which ferric iron chelated by the EDDHMA anion in any one or more of its ismoeric forms, and mixtures of such compounds. The compounds just mentioned are sometimes identified by names such as sodium ethylenediamine-N,N'-di-(o-hydroxyphenyl) acetato ferrate (III), which corresponds to the compound containing sodium ions and the ortho-ortho form of the chelated anion [Fe (III) EDDHA].sup.-.
2. Description of the Prior Art
Iron EDDHA and iron EDDHMA are known to be effective agents for remedying iron deficiency in agriculture, particularly in highly alkaline soils. In the past, powders or granules of iron EDDHA or iron EDDHMA have been applied directly to the soil or have been dissolved in irrigation water. Both compounds however suffer from the disadvantage of low solubility in water; the powder forms are particularly difficult to handle because of their tendency to cake or form lumps in the presence of water. Aqueous solutions of the compounds are correspondingly difficult to prepare.
Attempts to make such solutions tend to result in the precipitation of a sticky mass of material in the mixing vessel, unless the addition of the solid iron EDDHA or EDDHMA to the water is accompanied by continuous strong agitation of the contents of vessel. This causes a particular problem when it is desired to dissolve the compounds in irrigation water where agitation to said dissolution is difficult. Attempts to solve this problem by preparing aqueous solutions of the compounds for direct application to the soil have not been particularly successful because, even if sufficient agitation is used, it is not possible to obtain a solution which is more concentrated in iron EDDHA than about 30% w/v (i.e. about 30 g of iron EDDHA chelate in 100 cm.sup.3 of solution, an iron EDDHA concentration of about 300 g dm.sup.-3), or more concentrated in iron EDDHMA than about 20% w/v (i.e. about 20 g of iron EDDHMA chelate in 100 cm.sup.3 of solution, an iron EDDHMA concentration of about 200 g dm.sup.-3).
Since commercially available iron EDDHA and EDDHMA chelates are themselves only about 6% iron by weight and normally contain equimolar amounts of salts such as sodium sulphate or sodium chloride, a 30% w/v aqueous solution of iron EDDHA has an iron concentration of only about 1.8 g per 100 cm.sup.3 (about 18 g dm.sup.-3) and a 20% w/v aqueous solution of iron EDDHMA has an iron concentration of only about 1.2 g per 100 cm.sup.3 (about 12 g dm.sup.-3).
The low iron contents of these solutions, the difficulties in preparing the solutions and the problems which are associated with agricultural use of iron EDDHA and EDDHMA chelates in solid form (even after granulation, itself an expensive process) tend to restrict the agricultural usefulness of what might otherwise be an effective agent for remedying iron deficiency in soils. Similar problems are expected to arise in the agricultural use of other iron chelate compounds.